Image signatures are image features for identifying (determining the identity of) images. By comparing an image signature extracted from an image with an image signature extracted from another image, an identity scale (in general, referred to as similarity or distance) indicating a degree of the two images being identical can be calculated from a comparison result. Further, by comparing the calculated identity scale with a threshold, it is possible to determine whether or not the two images are identical. In this context, the meaning of “two images being identical” includes not only the case where the two images are identical at the level of image signals (pixel values of the pixels constituting the images), but also the case where one image is a duplicate image of the other by means of various alteration processes such as conversion of compression format of an image, conversion of size/aspect ratio of an image, adjustment of color tone of an image, various filtering processes (sharpening, smoothing, and the like) applied to an image, local processing (caption superimposition, cutout, and the like) applied to an image, and recapturing of an image. By using image signatures, as it is possible to detect duplication of an image or a moving image which is a set of images, for example, image signatures are applicable to an illegal copy detection system for images or moving images.
Patent Document 1 describes an example of an image signature. FIG. 18 is an illustration showing a method of extracting an image signature described in Patent Document 1. This image signature is a feature vector in multiple dimensions (sixteen dimensions in FIG. 18). The method includes respectively calculating average luminance from thirty two pieces of rectangle regions 244 (among them, sixteen pieces of rectangle regions are shown in FIG. 18) at predetermined positions in an image 240, and calculating differences in the average luminance between rectangle regions forming pairs (the paired rectangle regions are linked to each other with dotted lines 248 in FIG. 18), to thereby obtain a difference vector 250 in sixteen dimensions. With respect to the difference vector 250, a composite vector is generated by means of vector transformation, and a quantization index vector in sixteen dimensions, acquired by quantizing the respective dimensions of the composite vector, is used as an image signature.    Patent Document 1: Japanese Unexamined Patent Publication No. 8-500471